The present invention relates to a compressed air reservoir and an air dryer and a sweep air system for the dryer. In the illustrated embodiments, the invention is applied to an air braking system of a vehicle, such as a truck. Such a system typically includes an air compressor and a reservoir for storing the compressed air. Compressed air is drawn from the reservoir to provide for application of air brakes and/or other vehicle devices. Such a system also typically includes an air dryer for removing moisture from the compressed air. It is known to use a membrane dryer to remove moisture from the compressed air, and to then remove the moisture from the membrane dryer by a flow of sweep air across the membrane dryer.